I am troubleshooting a problem with one of my Aries keyboards -- namely that the calibration (1v/8ve) is not consistent across the keyboard. That is, if I adjust 1v/8ve tracking in the middle octave of the keyboard the other octaves are wider or smaller (wider for low voltage and smaller for high voltage).

The voltages come from a 60 resisters which are fed by a constant current source. The high end voltage is controlled by the calibration pot. I've check the constant current source and it is rock-steady so that rules out the current source.

The keyboard contacts 'tap' the resistor network and with no load (that is, when I disconnect the keyboard contact bus from the pcb, it taps the voltages correctly. Thus, the resistor network is fine.

However, when I connect the keyboard contact bus to the pcb, then the voltages are messed up (i.e. -- non linear).

The keyboard contact bus feeds a voltage follower -- the voltage follower, in turn, feeds a sample/hold. I have not yet checked the voltage follower.

The voltage follower is an old (1976 vintage) LM301 op amp. In the process of servicing the keyboard, the keyboard contact lead broke off and I had to resolder it. While not likely, it is possible that I overheated the op amp.

The reason I suspect the op amp is that it is pulling down the voltage from the keyboard -- the higher the voltage the greater the amount of "pull down".

If anyone has any ideas of other things for me to check, please let me know.

Sounds like the keyboard scaling is off.
Pull out the LM301a (IC9) and put a voltmeter to pin 3 and ground
Press the highest key, and adjust P1 for 5 volts, then press the keys at each octave and see if they are consistent at 1v per octave.
If they are, then the keyboard is fine, and the problem is probably on the circuit board, if they are not 1v per octave, then the problem may be the keyboard it's self.
If the keyboard voltages are fine, plug the LM301 back in and try it.
Let me know how you make out.

Do you have the schematics for it? if not let me know, I have them here.
I also have some spare lm301's if you need any.

Sounds like the keyboard scaling is off.
Pull out the LM301a (IC9) and put a voltmeter to pin 3 and ground
Press the highest key, and adjust P1 for 5 volts, then press the keys at each octave and see if they are consistent at 1v per octave.
If they are, then the keyboard is fine, and the problem is probably on the circuit board, if they are not 1v per octave, then the problem may be the keyboard it's self.
If the keyboard voltages are fine, plug the LM301 back in and try it.
Let me know how you make out.

Do you have the schematics for it? if not let me know, I have them here.
I also have some spare lm301's if you need any.

Dave

I already tested the keyboard voltages as you outlined above. Aries didn't provide IC sockets so I'll have to desolder the LM301a. Fortunately, I have a couple of LM301s here that I can try (I'll go ahead and put a socket on the board once I remove the old IC).

Though the 1/12 volt per semitone voltages were correct from the keyboard divider, the output voltages were not proportional to the keyboard voltages.

Why this problem seemed "impossible"

Normally, when dealing with op amps, the output voltage is proportional to the input voltage across the range. In other words, one would expect consistent voltage spacing across the keyboard even if the value was different from 1/12 volt per semitone.

However...

...the Aries keyboard's internal sample/hold utilizes a dual n-channel FET that feeds an op amp that is configured as a differential amplifier. If the dual FET's performance had degraded then it would be a source of non-linearity.

The fix

Turns out that through-hole Dual FETs are hard to come by these days so I decided I had nothing to lose by replacing the Dual FET (an E411) with a pair of BF245Cs (that I happened to have on hand).

The result

The keyboard tracking is now consistent across the range. Over the last few years I had noticed that the keyboard's tracking wasn't perfect but I had written the problem off to imperfect oscillator tracking. The top octave was always noticably 'off'. Now it is rock solid across the range. I suspect that I had experienced incremental degradation of the dual FET and it finally hit a point where it wouldn't track at all.

In the future if I encounter a similar error, I'll know to look for differential amplifier stages. _________________-- Kevinhttp://kevinkissinger.com

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